Data from: Wildfire provides refuge from local extinction but is an unlikely driver of outbreaks by mountain pine beetle

Bark beetle outbreaks and wildfire are important disturbances in conifer ecosystems, yet their interactions are not well understood. We evaluated whether fire injury increased susceptibility of lodgepole pines (Pinus contorta) to mountain pine beetle (Dendroctonus ponderosae Hopkins), how it influenced beetle reproductive success, and whether beetle population phase altered this interaction. Eight sites that experienced wildfire and eight unburned sites were examined in the Greater Yellowstone Ecosystem (U.S.A). Half were in areas where D. ponderosae was undergoing outbreaks and half were in areas with low populations. We examined 2056 trees one year after fire for burn injury and beetle attack. We quantified beetle reproductive success in a random sample of 106 trees, and measured gallery areas of D. ponderosae and competing subcortical herbivores in 79 additional trees. Baited flight traps sampled stand-level populations of subcortical herbivores and predators. Wildfire predisposed trees to D. ponderosae attack, but non-linearly, with moderately-injured trees being most preferred. This tree-level interaction was influenced by stand-level beetle population size, in that both healthy and fire-injured trees of all classes were attacked where populations were high, but no healthy trees, and only low- and moderately-injured trees were killed where populations were low. The number of adult brood produced per female was likewise curvilinear, being highest in moderately-injured trees. This reflected an apparent trade-off, with high intraspecific competition arising from the large number of beetles needed to overcome defenses in healthy trees, versus high interspecific competition and low substrate quality in more injured trees. These results suggest fire-injured trees can provide a reservoir for D. ponderosae during periods when populations are too low to overcome defenses of healthy trees, and might otherwise face localized extinction. However, the likelihood of populations increasing from endemic to outbreak levels in response to increased susceptibility is offset by the opposing constraints of lower substrate quality and higher competitor load in severely-injured hosts, and the relative scarcity of moderately-injured trees. Wildfire may confer some reproductive increases to populations already out-breaking. We present a conceptual model of how these disturbances and inherent feedbacks interact to affect beetle population dynamics.

树皮甲虫暴发与野火是针叶林生态系统中的重要干扰因子，然而二者的相互作用尚未得到充分阐明。本研究评估了火烧损伤是否会提升扭叶松（Pinus contorta）对山松甲虫（Dendroctonus ponderosae Hopkins）的易感性，探讨了火烧如何影响甲虫的繁殖成功率，以及甲虫种群阶段是否会改变这一相互作用。研究在美国大黄石生态系统（Greater Yellowstone Ecosystem）内选取了8个曾发生野火的样地与8个未受火烧的对照样地，其中一半处于山松甲虫暴发区域，另一半处于甲虫种群低密度区域。我们在火灾发生一年后对2056株树木开展了火烧损伤与甲虫侵袭情况的调查，通过对106株树木的随机抽样量化了甲虫的繁殖成功率，并在额外79株树木中测量了山松甲虫与其他竞争性皮下植食性昆虫的坑道面积，同时利用诱饵飞行诱捕器对样地内皮下植食性昆虫与捕食者的种群进行了样地水平采样。野火会使树木更易遭受山松甲虫侵袭，但这一关系并非线性，中度火烧损伤的树木最易被偏好。这一单株树木水平的相互作用受样地水平甲虫种群规模的影响：当甲虫种群密度较高时，所有损伤等级的健康树木与火烧损伤树木均会受到侵袭；而当种群密度较低时，无健康树木受到侵袭，仅轻度与中度火烧损伤的树木会被杀死。每头雌虫产出的子代成虫数量同样呈曲线分布，在中度火烧损伤的树木中达到峰值，这一现象反映了明显的权衡关系：在健康树木中，甲虫需要大量个体才能突破树木防御，因此种内竞争激烈；而在重度火烧损伤的树木中，种间竞争加剧且寄主基质质量下降。研究结果表明，在甲虫种群密度过低而无法突破健康树木防御的时期，火烧损伤的树木可作为山松甲虫的种群储备库，否则甲虫可能面临局域灭绝；然而，随着寄主易感性提升，甲虫种群从地方流行级别跃迁至暴发级别的可能性，会被两个相反的约束条件抵消：一是重度火烧损伤寄主的基质质量更低、竞争者负载更高，二是中度火烧损伤树木的相对稀缺性。野火或可为已经处于暴发阶段的甲虫种群带来一定的繁殖收益，本研究提出了一个概念模型，用以阐释这些干扰因子与内在反馈如何共同影响甲虫种群动态。